Is this surprising? Websites have long been silently writing to disk, for cache, cookies, and blobs. OPFS just provides a file-system-like API for ultimately the same functionality

> Without even asking whether the site can use local storage?

Where did you see this in the article? I had some recollection that Firefox at least did require asking the user.

That surprised me as well.

I thought the whole point of cookies, local storage, session storage, and indexed DB were to avoid what origin private file system is doing.

You mean I could have just saved stuff as a file this whole time instead of serializing it to a string? Why didn't we just do this from the start?

It depends what you mean by "general purpose." First, these things generalize more often than you'd expect. Second, even in the absence of generalization they're still useful for, e.g., fingerprinting activities to manufacture a unique ID where non previously existed.

That's basically just a research, theoretical attack vector. It doesn't mean it's viable for general purpose old school mass privacy invasion

There's no such thing as a sandbox "on your machine" when you really think about it. The code still runs on the same hardware and there are tons of ways to fiddle with said hardware that could be exploited (like rowhammer). The only "real" sandbox is fully dedicated hardware down to bare metal with zero connections to sensitive systems.

And now that Google's web environment integrity is getting repackaged into captchas, it seems we won't even be able to try to block such things in the future...

I got $HOME in a huge HDD because it was cheaper. I guess we belong to the cool kids club now?

I assume it's something like this:

Attacking website periodically makes random reads from a large file in localStorage. Other tabs and websites open have Javascript running that periodically performs operations that will result in SSD traffic. For example, GMail has a certain polling interval to check for new mail, and each request is going to result in a cache write that makes the SSD busy and delays other conflicting IO operations. Reddit checks for new chat messages. Large memory-heavy websites get paged out of RAM.

The pattern of IO operations that a website makes creates a fingerprint of interference with the IO ops that the attacking website is doing, showing up as differing amounts of latency as the SSD is periodically busy. This fingerprint can then be reconstructed to a specific website by training a CNN on it, basically using a neural net to classify a certain pattern of delays to the IO ops that other websites are doing.

In theory it makes sense, but it seems very noisy. Anything that makes absolutely zero requests or IO operations in the background (like say HN, or most old-school text sites) wouldn't show up, and would be indistinguishable from any other zero-request site. And having other sources of IOps on the same computer - say you're running an Ethereum client that's perpetually updating the blockchain, or you're downloading a bunch of torrents, or you've got DropBox and it's syncing your directory - would introduce noise that throws off the classifier.

That’s timing the cache, that’s old stuff by know. As I understand, this writes a relatively large file („Gigabytes“) using this OPFS api, which is different from the „localStorage“ api. This seems to use actual filesystem storage on the client, instead of living completely in memory (which may be reasonable given the size of files supported). This allows to actually time SSD IOPS latency by doing random reads.

Collected enough of these samples, together with the information of what else runs on the host, put that in the ML-Blender and the result will be able to tell you, with some accuracy, from a given set of samples, what’s running on the host.

I am sure i misunderstood some things because there are so many caches and unknowns in that setup that I struggle to understand how there could be any correlation, but that’s my understanding so far.